The present invention relates to a linking component that mainly comprises a nut/screw jack, and a robot arm fitted with said component.
An essential advantage of nut/screw screw jacks is that they can be used to transmit accurate movements between the two components they link while resisting significant force. More precisely, one of the linked components is connected to the screw and the other component to a housing covering the nut in which the said nut is fitted by bearings that allow it to rotate. The housing also incorporates a motor that reacts to external commands to rotate the nut and thus displace the screw in translation, thereby bringing the two components together or moving them away from one another.
Although the relative rotation of the screw and the nut along an axis shared with the axis of the screw causes the translation movement required, it is, however, essential that the nut be subject to no rotation along an axis at right angles to the axis of the screw, i.e. along any axis located in the plane of symmetry of the nut. If this occurs the long thin screw bears on two diametrically opposed points on the nut which becomes skewed. The compression force to which the screw is subject causes it to buckle and also causes significant premature wear on the points of contact.
A jack must fulfil two functions: it must guide mechanical parts in translation and it must bring the said parts further from or closer to one another. A screw/nut system easily performs the second function but is poor at achieving the first. Guiding the nut in translation relative to the screw implies exercising rotation at right angles to the axis of the screw, which rapidly deteriorates the device. A simple solution consists in adding other mechanical components that fulfil the translational guidance function. For example, a slider mounted on the end of the screw; this has the drawback of doubling the length of the screw. Another solution is to use a sliding pivot link fitted parallel to the screw, or more generally a prism-shaped link can be fitted coaxially to the screw. In this last configuration the screw is a ball plate, which has the drawback of significantly increasing the diameter of the jack and destroying its axial symmetry. All these solutions significantly increase the size and weight of the assembly; they also require that the guiding components connected to the screw be precisely aligned with the screw along its entire translation course, which is physically impossible to achieve with any degree of accuracy. The closer one gets to achieving this, the more machining becomes costly and the bulkier the parts become.
The ideal solution therefore consists in an assembly that only allows axial translation force between the screw and the nut. This assembly must allow rotation of the screw/nut on the same axis as the screw while prohibiting any screw/nut rotation at right-angles to the axis of the screw, i.e. the axis located in the plane of symmetry of the screw.
This twofold problem does not appear to have been resolved yet with manufacturers of nut/screw jacks going no further than acknowledging that their products are incapable of resisting significant transversal force, thereby limiting their use or making it necessary to use very thick screws to resist any flexion caused by flexion stress.
The linking component proposed here provides a solution to this twofold problem, thereby making it possible to use nut/screw jacks irrespective of the direction of the forces exerted between the parts it links while preventing screw-nut rotation in the axis at right angles to the axis of the screw. The size and weight of nut/screw jacks can thus be reduced relative to those of the known art for two different reasons:
there is no longer any need to choose a thick screw to resist flexion, and the linear guiding function between the screw and the sheath is omitted.
The linking component of the invention comprises a jack with a nut into which a screw is engaged, a sheath in which the nut is maintained by bearings such that it is free to rotate, a motor capable of producing rotation between the screw and the nut, and first and second linking means between the first component and the nut and between the second component and the screw. It is characterized in that the first and second connecting means consist of universal joints that each permit two axes of rotation perpendicular to the screw and the nut, and the axes of rotation of one of the universal joints converge towards the center of the nut. xe2x80x9cUniversal jointsxe2x80x9d are understood to mean transmission mechanisms with two joints, of which the hinge pins are fitted respectively to the parts connected by the universal joint (one of the two parts mentioned above, and the screw or nut) and are linked to one another by an intermediate component that maintains them perpendicular to one another. The main function of the universal joints is to transmit rotations between the components they connect while allowing for variations in the angle between the parts.
This type of linking component may be used in a robot arm, particularly in a connection between two consecutive sections that are hinged so as to form a variable angle, the axis of the hinge being perpendicular to the said sections; the jack is fitted between two lever arms of the said sections and brings them further from or closer to one another.
The linking component is particularly useful in narrow robot arms that are expected to pass through narrow openings, for example through protective housings. It is well known that the thickest parts of such arms are where two sections are hinged; this phenomenon is largely overcome in this invention, thereby retaining the narrowness of the arms.
Another advantageous characteristic of the invention is the resulting saving in weight, which becomes crucial when several components fitted with this type of joint need to be connected end to end. When the number of components is increased, the forces to which the first components in the chain are subjected rapidly become so great that this type of structure must in practice be limited to a very small number of sections.
Those skilled in the art will understand that, in order to be fine and dexterous, the structure of a mechanical arm must allow for the coexistence, and if possible the alternation, of a large number of components connected in sequence by two types of joint:
one type allowing rotation around the axis of the previous sections (the roll axis) and the other allowing rotation around an orthogonal axis (the pitch axis), the latter being slightly off-center relative to the axis of the previous component. The problems of pitch rotation have hitherto limited this type of structure to a limited number of components, not only because of the excessive thickness mentioned above, but also because of the rapid increase in weight, and hence the force required, as the number of sections grows.
The invention thus makes it possible to construct a fine, dexterous arm capable of passing through small-diameter openings and comprising more than three modular components including:
a narrow hollow section, which is advantageously cylindrical or prismatic in cross-section,
a mechanical device advantageously positioned at the distal end of the section and allowing rotation around the axis of symmetry of the section or a longitudinal axis,
a mechanical device allowing rotation around an axis orthogonal to the previous axis and advantageously offset, this section using the jack described above, and connecting this modular component to the next in the sequence.
The order in which these components are mounted may be reversed without going outside the scope of the invention.
If the number of modules mounted end to end increases, the power developed in the jack must be increased slightly, which somewhat limits the modularity of the invention; however, slight changes of power make it possible to build an arm comprising three or more of these components.
Another aspect of the invention is the measurement of forces. This is particularly desirable for dexterous arms operating in hostile environments, firstly to give accurate control over the maneuvers to be carried out, and secondly because, given the great reversible capacity of the jack, any collisions can easily be detected; this information is very useful for safety purposes.
The jack of the present invention can be used to create an assembly that only allows axial translation force between the screw and the nut. This has a secondary advantage in that it allows for very easy measurement of the traction or compression forces exerted by the jack without any interference from other mechanical forces. All that is required is to measure the traction or compression forces exerted on the screw itself. In a preferred embodiment a flat spot is machined onto the screw itself in the region of the fastening-point and two or four extensometers are bonded onto the flat spot. The measurements taken in this way are remarkably free from interference.
The same principle, of isolating the mechanical force to be measured, has been extended to the other joints of the arm, i.e. those that allow for roll movement. To do this, an additional device is used to separate the torque transmitted from any other mechanical force, thereby excluding any interference. This device consists of an incoming pin connected to a disk-shaped component, and a outgoing pin connected to another disk-shaped component. The two disks are placed facing one another and each is fitted by means of a ball-joint with a small connecting rod to which the transferred torque applies pure traction or compression stresses. This is achieved by locating the connecting rod in a plane that is precisely perpendicular to the axis along which the torque is transmitted and in an orthoradial direction. This assumes that the two disk-shaped components are fitted with a projection to position the ends of the connecting rod accurately. The connecting rod can then be fitted with any type of stress-measuring apparatus, a preferred embodiment consisting of two or four extensometers bonded onto the connecting rod.
The invention also relates to a modular robot arm structure consisting of at least three components of virtually identical diameter. Each of these components comprises a section, a joint allowing for roll rotation coaxial to the section, and a joint allowing for pitch rotation orthogonal to the axis of the section and offset from the section. Most of the linking section described above may essentially be contained inside the sections and constitute the pitch joint.